1. Field of the Invention
The invention relates to a method for truncating engine torque in a four-wheel drive vehicle powertrain to regulate maximum vehicle wheel traction.
2. Background Art
In a typical automatic four-wheel drive vehicle powertrain, engine torque is transmitted to vehicle traction wheels through a transmission and through a torque transfer case situated at the torque output side of the transmission. Such vehicles have two driving axles, one at the rear of the vehicle and one at the front. The transfer case divides driving torque proportionally between the front axle and the rear axle in accordance with a control algorithm residing in a four-wheel drive vehicle system control module.
During one operating mode, the control module will condition the powertrain for torque transfer from the torque transfer case to the rear axle through a rear wheel driveshaft, thus establishing normal rear-wheel drive operation. In another operating mode, the driving torque is divided into a first torque component distributed through a front wheel driveshaft to the front axle and a second torque component distributed through the rear wheel driveshaft to the rear axle.
The control module, which receives input data from speed sensors that measure the speed of each axle, as well as other input variables, uses those measurements as input data to execute the control algorithm to determine how much torque should be distributed to each axle. The controller will cause a transfer of additional torque to one axle and reduce torque at the other axle depending upon the traction wheel torque loads.
It is known design practice to effect a four-wheel driving mode by locking the rear traction wheels to the front traction wheels using a positive drive locking clutch in the torque transfer case. Typically, when the positive drive locking clutch is engaged, provision is made for changing the driving speed ratio between a direct drive speed ratio and an under-drive speed ratio.
It is known design practice also to eliminate the positive drive locking clutch and to establish instead a positive driving connection between the driving axles using a friction clutch with torque input friction elements connected driveably to one driving axle and companion torque output friction elements connected driveably to the other driving axle. The friction clutch, under normal operating conditions, can be commanded to achieve its maximum torque transmitting capacity to effect a locked driving connection of one axle to the other. If the transfer case is not sized to reach a traction wheel skid torque, however, it is possible under some rare operating conditions for the clutch to slip and to damage the clutch friction elements. This could occur, for example, if the vehicle is operating on an off-road terrain or during certain winter driving events when the output speed of the rear axle will exceed the output speed of the front axle, thus creating a clutch slip condition that can cause clutch failure if the slip condition continues, even for a short time. Operation of the vehicle under this condition causes the rear traction wheels to spin when the front traction wheels do not spin. This typically would occur, furthermore, when the engine throttle is at an advanced setting.
If the clutch were to be designed with sufficient capacity to effect a positive lock between the driveshafts for the front traction wheels and the rear traction wheels under all four-wheel driving conditions, the required geometry of the clutch, of necessity, would make it difficult, if not impossible, to meet the packaging constraints of a contemporary four-wheel drive vehicle powertrain. Further, each of the components that distribute torque from the engine through the transfer case to the front axle would be required to accommodate the peak torque associated with a locked up friction clutch. This would require a design upgrade for the torque transmitting components of the transfer case that would be in excess of the design requirements for normal operating conditions of the vehicle even though skid torque of the front traction wheels is rarely experienced.